1. Field of the Invention
The present invention relates to a nonvolatile semiconductor device and manufacturing method therefor.
2. Description of the Related Art
Nonvolatile memories such as NAND and NOR memories suffer the problem that a threshold Vth for operating a transistor varies in repetitive read/write of data. This problem becomes actualized along with reduction in the feature size of elements, and readily occurs when the film thickness of a gate insulating film is 80 Å or less.
To solve this problem, a multilayered structure using a passivation film has conventionally been adopted. In this multilayered structure, as shown in FIG. 14, a plasma-enhanced-SiON film 51 is formed on an upper metal wiring (e.g., Al-0.5 at % Cu) 50. A plasma-enhanced-SiN film 52 is formed on the SiON film 51. The SiN film 52 is used as a measure against moisture absorption, and the SiON film 51 is used to cut off hydrogen contained in the SiN film 52.
FIG. 15 shows data as a result of a comparison between a shift amount ΔVth in a structure having a passivation film and a shift amount ΔVth in a structure having no passivation film. The structure having a passivation film means a structure in which the SiN film 52 and SiON film 51 shown in FIG. 14 are stacked. As shown in FIG. 15, the structure having a passivation film exhibits a larger shift amount ΔVth than the structure having no passivation film.
The prior art cannot essentially suppress the shift ΔVth in an element which repeats data read/write even with a multilayered structure having a passivation film.